"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Polysilanes 187
Figure 8.1 shows the GPC analysis of polysilane 2 and its radical-based
degradation [12]. Line A shows a typical bimodal distribution
(MW ¼ 2320, MW=Mn ¼ 1:67 against a polystyrene standard), the low molecular
weight portions being mainly cyclic. Lines B and C show the outcome of
the reaction between thermal generated t-BuO: radical with 2 at 50 8C/5 h
and 140 8C/1 h, respectively. The MW of the starting materials decreased
from 2320 to 840 and 750, respectively, whereas the polydispersity index
(Mw=Mn) decreased from 1.67 to 1.28 in both experiments. In the absence of
radical initiators, any decomposition of polysilane 2 was found upon heating.
Radical inhibitors such as quinones, considerably slow down the polysilane
degradation, thus indicating that silyl radicals are involved as
intermediates.
It was proposed that an initially generated silyl radical 3, by reaction
of t-BuO: radical and polysilane 2, attacks another silicon atom in the
same backbone to give a cyclic polysilane that contains an acyclic chain and
another silyl radical (Scheme 8.1) [12]. The last silyl radical can either cyclize
or abstract a hydrogen atom from another macromolecule, thus propagating
the chain degradation. The reaction in Scheme 8.1 is an example of intramolecular
homolytic substitution (SHi), a class of reactions discussed in
Chapter 6.
% Weight (Arbitrary Units)
B
C A
102 103 Molecular Weight
Figure 8.1 Gel permeation chromatograms related to the radical initiated degradation of
poly(phenylhydrosilane) (2). (A) Initial sample with Mw ¼ 2320 and Mw=Mn ¼ 1:67. (B)
Sample obtained after 5 h heating at 50 8C of a benzene solution containing 0.09 M of 2 in
the presence of t-BuONNOBu-t (10 mol %); Mw ¼ 840 and Mw=Mn ¼ 1:28. (C) Sample
obtained after 1 h heating at 140 8C ofatert-butylbenzene solution containing 0.09 M of 2
in the presence of t-BuOOBu-t (10 mol %); Mw ¼ 750 and Mw=Mn ¼ 1:29.
10 4